MD82 FLC HESITATED IN TAXIING INTO POS FOR TKOF CAUSING THE TWR CTLR TO SEND AN ACFT ON FINAL AROUND.

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WHILE TAXIING FOR TKOF AT DCA ARPT; WE WERE GIVEN A RE-ROUTE TO STL. AFTER SETTING UP THE RADIOS FOR THE NEW ROUTE; WE ADVISED THE TWR CTLR THAT WE WERE READY FOR TKOF; ON RWY 1 AT DCA. WE WERE ISSUED A CLRNC TO TAXI INTO POS AND HOLD ON RWY 1 AND THAT THERE WAS TFC (AT OR INSIDE) THE OUTER MARKER. I CAN'T REMEMBER THE EXACT WORDING. IT TAKES THE ENG ON THE MD80 A FEW SECS TO SPOOL UP TO START TO MOVE AND WE WERE NOT RIGHT UP TO THE HOLD SHORT LINE; SO IT TOOK A FEW SECS TO GET TO THE RWY TWR THEN ISSUED A CLRNC TO 'HOLD IN POS; TFC ON A 4 MILE FINAL.' THE CAPT; THE PF; DID NOT UNDERSTAND THE LAST PART OF THE TRANSMISSION AND WAS UNSURE WHETHER TO TAXI INTO POS AND HOLD NOW OR HOLD 'OUR' POS FOR THE INCOMING TFC; SO HE STOPPED THE ACFT SHORT OF THE RWY. HE WANTED ME TO CLARIFY OUR INSTRUCTIONS WITH THE CTLR; WHO WAS VERY BUSY. BEFORE I COULD CALL; THE CTLR ISSUED A CLRNC FOR TKOF AND SOUNDING SORT OF ANXIOUS VOICE SAID 'DON'T STOP.' BUT WE HAD ALREADY STOPPED. AFTER THE TKOF CLRNC WAS ISSUED; THE CAPT STARTED TO SPOOL THE ENG UP AGAIN AND THE ACFT STARTED TO MOVE ACROSS THE HOLD SHORT LINES. THE CTLR THEN ISSUED A CLRNC TO HOLD SHORT OF RWY 1 FOR LNDG TFC. I TRANSMITTED THAT WE WERE TO 'HOLD SHORT' BUT WE WERE ALREADY ACROSS THE HOLD SHORT LINES. THE CAPT TRANSMITTED THAT WE WERE NOW UNABLE AND GAVE THAT TFC MISSED APCH INSTRUCTIONS. WE WERE THEN ISSUED A TKOF CLRNC WITH NO OTHER PROBS. THIS COULD HAVE BEEN PREVENTED BY THE CAPT HEARING THE FIRST INSTRUCTIONS AND IF THE CTLR SPOKE SLOWER AND MORE CLEARLY.